DESCRIPTION The invention relates to a method for extracting fossil fuels of an offshore plant arranged in the water having at least one pipeline system, via which the fluid containing at least portions of the fossil fuels is extracted from a subsurface. A thermal energy contained in this fluid is extracted at least partially by means of a heat exchanger from the fluid and transferred to the water.

Presently usual methods of supplying oil platforms or offshore plants can be divided into two major groups. A so-called onshore electric generation includes the usage of power stations, which are placed on the mainland and sea cables are used for energy transmissions to consumers in the sea. This approach is described both by Myhre, Jorgen, "Electrical power supply to offshore oil installations by HVDC", and Hoerle, Nils et al., "Electrical supply for offshore installations made possible by use of VSC technology". Alternating current power supply becomes impracticable for long distances of more than 50 or 100 kilometers. High- voltage direct current (HVDC) technology is targeted to solve this problems, but electricity generation for platforms which are more remote from the shoreline is problematic due to energy losses. In addition further disadvantages of HVDC result in the high efforts for switching, controling, availability, conversion and maintenance.

Almost all power on offshore installations has been generated locally by gas turbines or diesels with low efficiency and high green gas emissions as a result. Platforms on the Norwegian shell e.g. contribute 25 % of the nation's over all carbon dioxide emissions. Both carbon dioxide and nitrogen oxide emissions arise by operating gas turbines or diesels for the purpose of supplying offshore platforms with energy. An alternative solution for offshore power generation can be based on the usage of wind energy. Both the usage of wind energy and other renewable power supply systems such as photovoltaic modules belong to the second major group of methods for supplying oil platforms with energy. This second major group is referred to as "offshore electric

3AMEHflK)iiiMH JIHCT generation". While wind energy and photovoltaic systems correspond to a surface generation, subsurface generations include various approaches of hybrid power sources or water current turbines. Hybrid systems use energy conversion devices with high specific power to efficiently achieve high levels of current, and energy storage devices with high specific energy to enable sustained operation. There exist different approaches and conceptual hybrid systems which have been identified as candidates for powering subsurface oil and gas production operations, such as e.g. the usage of a pressurized- water nuclear reactor in combination with a lead-acid battery. While onshore electric generation is accompanied by substantial losses of energy which effect the more serious the longer the supply lines are, also offshore surface electricity generation bares several problems. Surface generation systems such as wind energy devices are exposed to the weather, which means that they are prone to malfunctions when a storm arises or due to the influence of a wind calm, which is accompanied by an irregular energy supply. Furthermore, cloudy conditions prevent a steady energy supply when photovoltaic devices are in use.

Therefore, it is the object of the present invention to provide a particularly effective and at the same time particularly reliable method of the initially mentioned kind. This object is solved by a method having the features of claim 1 and by an offshore- plant having the features of claim 9. Advantageous configurations with convenient developments of the invention are specified in the dependent claims.

In the method according to the invention, the thermal energy extracted from the fluid is used for generating mechanical energy.

Due to the steady fluid flow an almost constant amount of thermal energy can be extracted from the fluid. Therefore the thermal energy derived from the fluid is a very reliable energy source, which is almost independent from weather conditions. In other words the pipeline system which contains the fluid from which the thermal energy is extracted is not exposed to the weather due to the fact that it is surrounded by water. In summary a very constant amount of thermal energy can be extracted from the fluid

3AMEHSK)ntHM JIHCT which is especially almost independent from boundary conditions like weather influences. This is particularly advantageous in comparison to surface generation of energy by operating wind turbines or photovoltaic systems, which are extremely dependent from the weather conditions whereat they can take serious damage since they are exposed to sea storms.

In an advantageous configuration the water heated by the transfer of heat is used at least partly for driving at least one turbine wheel, wherein by means of a drive shaft, which is connected with the at least one turbine wheel in a torque-transmitting way, mechanical energy is transmitted.

The heated water has a smaller density than the surrounding cold water. Therefore, the heated water gains a buoyancy, which is accompanied by an upwards directed water flow. The kinetic energy of this water flow can be converted into mechanical energy in a very efficient way, if the water flow is used to drive at least one turbine. This is especially advantageous, since mechanical energy is produced, without emitting any gases, which are harmful to the environment. Furthermore, the mechanical energy can be transmitted to respective consumers with very low losses of energy, whereat a combination of shafts and a gearing can be used, to provide the mechanical energy according to the requirements.

It has proven further advantageous, if the water heated by the transfer of heat is used at least partly for operating a power machine, which works in a Stirling cycle process and operates the drive shaft.

Stirling engines are especially designed for converting waste heat into mechanical energy. Operating a power machine is - as well as operating a turbine wheel as described above - carbon neutral and therefore it is also not harmful to the environment. In addition it is very quiet and very reliable in combination with low maintenance efforts especially in comparison to internal combustion engines or gas turbines which are used for providing energy to the offshore plant in a conventional way.

3AMEHflK)i HH JIHCT Furthermore, it is particularly advantageous if by the mechanical energy at least one electric generator is driven.

In comparison to other forms of energy, electric energy requires the lowest expenditure on equipment since only cables or e.g. current converters are required to transfer the electric energy from the source to the electrical consumers. Furthermore, an electric generator is very suitable, since it is capable to directly convert mechanical energy e.g. in the form of rotational energy into electric energy which can be stored in batteries in addition.

It is further advantageous, if at least parts of the electric energy generated by the at least one generator is used for the energy supply of a drilling platform of the offshore plant.

Instead of taking high efficiency losses into account, which occur, when long distances have to be bridged and accordingly long cables have to be used to connect an energy source which is e.g. situated onshore with an oil platform, especially short cables can be used, if the electric generator is situated close to the drilling platform respectively close to the offshore plant. In other words especially in comparison to onshore electric supply the losses of energy can be reduced to a minimum by using very short cable lengths. A drilling platform which corresponds to an oil platform carries many electrical consumers, which have to be supplied with different amounts of power. While on the one hand supplying e.g. the living quarters for the operating personal or e.g. systems for lighting, heating and ventilation require a small amount of power, which can be provided by the generator, also systems with a higher demand of power, e.g. water injection systems or oil export systems can be supplied.

It has proven further advantageous, if at least parts of the electrical energy generated by the at least one generator is used for the energy supply of at least one device arranged under water, in particular on the subsurface.

Since the generator is close to the subsurface equipment, which has to be supplied with electricity the required cable length is accordingly short. In other words besides the

3AMEHflK)i¾HH JIHCT advantage of low energy losses, there is only little intervention in nature, since the lying of long cables on the seabed becomes superfluous.

It has proven further advantageous, if the water heated by the transfer of heat is conducted by means of an, in particular tube-shaped, duct device.

By means of a duct device the heated water can be guided directly to the energy converting unit, e.g. a turbine wheel, without being carried away by the sea current. In other words, a duct device can be used to increase the efficiency of the flow of the heated water onto the turbine. Thus both flow losses and heat losses can be decreased. A tube-shaped duct device is very suitable, since it has a round contour like the turbine wheel, so the heated water flows against the turbine wheel very favorable, whereat especially low flow losses arise. It has proven further advantageous, if components contained in the fluid are at least partly deposited on the walls of the heat exchanger facing the fluid, wherein components have a smaller average size as the fluid enters the heat exchanger than they have as the fluid exits the heat exchanger. The solved particles in the fluid form agglomerates of bigger average size, if the fluid is cooled down within a heat exchanger. In other words the heat exchanger is used to increase the average size of the dissolved particles whereat at the same time the number of particles is decreased, due to the fact that the small dissolved particles stick together and form agglomerates of bigger average size, which corresponds to a precipitation of particles. The precipitation is advantageous, because the deposition of particles with bigger average size within the pipeline system is not as pronounced as the deposition of particles with smaller average sizes. Since the particle deposition within the pipeline system is accompanied by a cross section reduction of the pipe, the decrease of particle deposition is important to keep the flow losses low.

The offshore plant according to the invention serves for extracting fossil fuels. The offshore plant has at least one pipeline system, via which a fluid containing at least parts

3AMEHSI0IUHH JIHCT of fossil fuels is capable of being extracted from the subsurface. The offshore plant comprises a heat exchanger, by means of which a thermal energy contained in the fluid is capable of being at least partly extracted from the fluid and transferred to the water. . The offshore plant comprises means for generating mechanical and/or electrical energy utilizing the thermal energy.

By the employment of the offshore plant short power lines can be used to cover at least parts of the required electrical energy to supply electrical consumers which are needed to operate the offshore plant. Furthermore, by means of the offshore plant electrical energy can be produced without emitting green house gases.

The advantageous and preferred embodiments described for the method according to the invention also apply to the offshore plant according to the invention and vice versa. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or alone without departing from the scope of the invention.

Further advantages, features and details of the invention are apparent from the claims, the following description of preferred embodiments as well as based on the drawings.

The drawings show in:

FIG 1 a heat exchanger according to the prior art which is connected with a pipeline whereat a fluid which contains dissolved particles flows through the heat exchanger; FIG 2 a perspective view of the heat exchanger according to the prior art which can be used to precipitate particles, which are dissolved in the fluid;

3AMEHflK)i¾HH JIHCT FIG 3 an electric generator which is driven by a convectional water flow caused by the heating of water by means of the heat exchanger, whereat the fluid, flows through this heat exchanger;

FIG 4 a schematic illustration of a Stirling engine, with a shaft, whereat the

Stirliing engine is driven by a thermal energy of the convectional water flow, and in

FIG 5 a schematic illustration of the electric generator, which is part of an offshore plant, whereat the electric generator is used to supply at least parts of electric consumers of the offshore plant with electrical energy.

FIG 1 shows a heat exchanger 8 which is surrounded by water 1. The heat exchanger 8 is integrated to a pipeline 3. By means of the pipeline 3 a fluid, which corresponds to oil 2 is pumped out of a subsurface 5. The oil 2 contains dissolved particles which correspond to dissolved wax 5. According to the direction of an arrow, which schematically shows a flow direction 7 the oil 2, which contains the dissolved wax 5 enters the heat exchanger 8. Before entering the heat exchanger 8 the oil 2 is warm in comparison with the water 1 Due to the heat transfer within the heat exchanger 8 the oil 2 is cooled, whereat the water 1, which surrounds the heat exchanger 8 is heated. The cooling of the oil 2 causes a precipitation of the dissolved wax 5, whereat precipitated wax 6 is created. In other words the dissolved wax 5 sticks together due to the cooling of the oil 2 within the heat exchanger 8 and forms the precipitated wax 6. The process of cooling down the oil 2 for the purpose of converting the dissolved wax 5, with wax particles of small average size into the precipitated wax 6 with particles with bigger average size within a heat exchanger 8 is known as cold seeding corresponding to the prior art.

The heat exchanger 8, which is appropriate in terms of the implementation of the cold seeding concept, is shown in FIG 2. According to the flow direction 7, which is marked by an arrow, the oil 2, which contains the dissolved wax 5 flows through an inlet 9 of the heat exchanger 8. The inlet 9 is both connected with the heat exchanger 8 and with

3AMEHflK)¾HH JIHCT the pipeline 3. The heat exchanger 8 is used to transfer the heat from the oil 2 to the water 1, which surrounds the heat exchanger 8. As a result of the heat transfer the cooled oil 2 exits the heat exchanger 8 and flows through an outlet 10, which is connected both with the heat exchanger 8 and the pipeline 3. The heat transfer from the oil 2 results in a convectional water flow 12, which is directed upwards and drives a water turbine 16, which corresponds to a turbine wheel. The functioning of a cold flow electric generator 17 is described in FIG 3. According to FIG 3 the water turbine 16 is connected with a power generator 18 by a shaft 19. The shaft 19 corresponds to a drive shaft. The operation of the heat exchanger 8 has been substantially described in FIG 1, and therefore in the following only the difference shall be presented. A hot oil 14, which corresponds to the oil 2 before entering the heat exchanger 8, and a cooled oil 15, which corresponds to the oil 2 after exiting the heat exchanger 8, leaves the heat exchanger 8. The difference of the thermal energy between the hot oil 14 and the cooled oil 15 is substantially transferred to a cold water 11 which flows over the heat exchanger 8 as a consequence of the upwards directed convectional water flow 12, which corresponds to a cold flow. In other words the cold water 11 has as well as the surrounding water 1 a lower temperature and therefore a higher density than the convectional water flow 12, which is generated due to the heat transfer from the heat exchanger 8. As a consequence a suction effect causes the cold water 11 to enter a duct device 25, which surrounds both the heat exchanger 8, the water turbine 16, the shaft 19 and the power generator 18. Since it is the object of the duct device 25 to ensure a substantially undisturbed incoming flow of the convectional water flow 12 to the water turbine 16 it is useful that the heat exchanger 8 is on the one hand situated perpendicular below the water turbine 16 and on the other hand and accordingly a longitudinal axis of the duct device 25 is also situated in perpendicular direction below the water turbine 16. According to the invention a cold flow electric generator 17 comprises the power generator 18, which is driven by the convectional water flow 12. Since not the whole energy of the convectional water flow 12 is converted into electric energy a heated water 13 corresponds to the convectional water flow 12 after the energy release of at least parts of the included energy to the water turbine 16. In other words the amount of energy of the convectional water flow 12 which was not released to the power generator 18 in the form of mechanical energy is substantially included in the heated water 13. Hence it

3AMEHflK)i¾HH JIHCT makes sense to use the remaining energy of the heated water 13 to operate e.g. a power machine which works in the Stirling cycle process. Therefore the power machine corresponds to a Stirling engine 20. The Stirling engine 20 can both be driven by the heated water 13 which contains the rest of the energy of the convectional water flow 12, which was not released to the water turbine 16 and the Stirling engine 20 can also be driven as a substitute of the water turbine 16, as it is schematically shown in FIG 4. Thereby the Stirling engine 20 converts at least parts of the energy of the convectional water flow 12 into mechanical energy, which is transferred to the power generator 18, which is not shown in this figure, by the shaft 19.

FIG 5 schematically shows an offshore plant 26 with an oil platform 24, which corresponds to a drilling platform, and a pipeline 3, which is used to convey oil from the subsurface 4 to the oil platform 24. A subsurface equipment, which corresponds to at least one device 23, is used to enable the conveyance of the oil 2 within the pipeline 3 or respectively to support e.g. the drilling work. The cold flow electric generator 17 generates electric energy by using the waste heat of the oil 2, and a cable 21 connects the cold flow electric generator 17 with the subsurface equipment 23 and thereby the subsurface equipment 23 can be supplied with electricity. Another cable 22 connects the cold flow electric generator 17 with the oil platform 24 and thereby at least parts of the electrical consumers upon the oil platform 24 can be supplied with electricity from the cold flow electric generator 17.

3AMEHfiK)IU JIHCT